1. Field of the Invention
The present invention relates to energy shifting arrangements. More specifically, the present invention relates to optical parametric oscillators (OPOs) used in eye-safe lasers and other devices.
2. Description of the Related Art
Lasers are currently widely used for communication, research and development, manufacturing, directed energy and numerous other applications. One particularly useful application involves target designation. Laser light is intense and collimated. Accordingly, lasers are favored for target designation inasmuch as they allow a forward observer to designate a target from a considerable and therefore, typically, safe distance with considerable accuracy. That is, since the beam is collimated, it does not spread unacceptably and the intensity of the beam remains high for a considerable distance. Laser target designators have been proven on the battlefield to be a useful aid in the accurate delivery of weapons to target.
An important requirement for target designation and other applications is that the laser be xe2x80x98eye-safexe2x80x99. For example, Neodymium-Ytterbium Aluminum Garnet (Nd:YAG) lasers are commonly used for numerous applications including laser target designation. Unfortunately, Nd:YAG devices normally lase at a wavelength of 1.06 xcexcm (microns), which is harmful to the eye.
Because only a few crystals lase and each lases at a unique fundamental frequency, OPOs have been widely used in designators and other devices to shift the fundamental output of a laser from one wavelength to another. For example, optical parametric oscillators (OPOs) have been used to shift laser output from the harmful wavelength of 1.06 microns output by Nd:YAG lasers to the eye-safe range of 1.53 microns.
OPOs use a nonlinear crystal to effect a shift of energy from one part of the spectrum to another. One frequently used crystal is potassium titanyl arsenate (KTA). Unfortunately, the use of OPOs limits the efficiency of the system. This is due to the fact that the energy in the input laser beam is split between plural output beams, only one of which is xe2x80x98eye-safexe2x80x99. This operates as a system constraint in many applications. For example, in the target designation application, the inefficiencies associated with the OPO conversion process adversely impact the size, cost, power requirements, weight and range of the system.
Hence, there is a need in the art for a more efficient system and method for shifting the energy output of a laser by other wavelength conversion pathways.
The need in the art is addressed by the system and method of the present invention. In a most general sense, the invention is an arrangement comprising a mechanism for shifting energy received at a first wavelength and outputting the shifted energy at a second wavelength. The second wavelength is a primaly emission capable of inducing a secondary emission of energy in the shifting medium. Hence, a novel feature of the invention is the inclusion of a second mechanism, in functional alignment with the first mechanism, for minimizing the secondary emission. This constrains the energy to be output by the arrangement at the desired wavelength.
In the illustrative embodiment, the first mechanism is an optical parametric oscillator having a crystal such as potassium titanyl arsenate. The crystal may be X-cut, Y-cut, etc. The second mechanism then includes first and second reflective elements. The first element is a first surface reflective at the second wavelength with a predetermined reflectivity at a wavelength of the secondary emission. Similarly, the second element is a second surface at least partially transmissive at the second wavelength with a predetermined reflectivity at a wavelength of the secondary emission. Optimally, the predetermined reflectivity of the secondary emission from both mirror surfaces is less than or equal to five percent.
In one embodiment illustrated herein, the first wavelength is approximately 1.06 microns, the second wavelength is approximately 1.53 microns and the secondary emission includes energy at 2.59 microns and 3.76 microns.
The present teachings enable the design and construction of efficient eye-safe lasers and other devices as will be appreciated by those of ordinary skill in the art.